Table 2.

Direct and indirect (secondary) signs of acute appendicitis in graded-compression, real-time US, colour Doppler and contrast-enhanced US (CEUS; adopted according to references 7, 9, 20 and 21)

Real-time US signs of acute appendicitis
Direct signs	Indirect signs
Non-compressibility of the appendix  Perforation: appendix might be compressible	Free fluid surrounding appendix
Diameter of the appendix > 6 mm	Local abscess formation
Single wall thickness ≥ 3 mm	Increased echogenicity of local mesenteric fat
Target sign:  Hypoechoic fluid-filled lumen  Hyperechoic mucosa/submucosa  Hypoechoic muscularis layer	Enlarged local mesenteric lymph nodes
Appendicolith: hyperechoic with posterior shadowing	Thickening of the peritoneum
Colour Doppler and contrast-enhanced US:  Hypervascularity in early stages of AA  Hypo- to avascularity in abscess and necrosis	Signs of secondary small bowel obstruction